To determine whether the extent of myocardial infarction differs in conscious baboons and pigs, both devoid of preformed collaterals, 40 min and 90 min coronary artery (CA) occlusion (O) both followed by 4-6 days of CA reperfusion (R) were examined in both species. In pigs and baboons, respectively, CAO reduced subendocardial blood flow to 0.01q0.01 and 0.03q0.01 ml/min/g and also reduced subepicardial blood flow to 0.08q0.01 and 0.05q0.01 mi/min/g. With CAR, LV diastolic pressure rose by 53q4.9% in pigs, but fell in baboons. Infarct size/area at risk (IS/AAR) was significantly greater (p<0.05) in pigs (49q6%) than in baboons (21q4%) with 90 min CAO. With 40 min CAO, IS/AAR was 46q3.6% in pigs, while in baboons the IS/AAR was minimal, i.e., 2q1%. Thus, both pigs and baboons were characterized by minimal coronary collateral circulation, but IS in conscious baboons was significantly less, as compared with conscious pigs. Potentially, these differences could be explained, in part, by natural protective mechanisms and/or less reperfusion injury in primates. These results in primates may also help to explain the salutary effects of CAR in patients at intervals